Ed. /Psych. 
Lib 


Gidffing 

On  the  Conditions  of 
Fatigue  in  Reading 


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THE  LIBRARY 
OF 

THE  UNIVERSITY 

OF  CALIFORNIA 

LOS  ANGELES 


TK, 


Vol.111.    No.  5. 


REPRINTED  FROM 


THE 


LIBRARY 

September,  1896. 


Psychological  Review 


J.  McKEEN  CATTELL 
Columbia  University 


EDITED  BF 

AND 


J.  MARK  BALDWIN 
Princeton  University 


WITH  THE  CO-O^Mi-RATION  OF 

ALFRED  BINET,  ficoLE  des  Hautes-£tudes,  Paris;  JOHN  DEWEY,  University  of 

Chicago;    H.  H.  DONALDSON,    University  of  Chicago;  G.  S.  FULLERTON, 

University  of  Pennsylvania;   WILLIAM  JAMES,  Harvard  University; 

JOSEPH  JASTROW,  University  of  Wisconsin  ;   G.  T.  LADD,  Yale 

University;  HUGO  MONSTERBERG,  Harvard  University; 

M.  ALLEN  STARR,  College  of  Physicians  and  Surgeons, 

New   York;  CARL  STUMPF,  University,  Berlin; 

JAMES  SULLY,  University  College,  London. 


CONTENTS. 

Studies  from  the  Psychological  Laboratory  of  the  University  of  Iowa : 

On  the  Effects  of  Loss  of  Sleep :  G.  T.  W.  Patrick,  and  J.  Allen  Gilbert,  469. 

Studies  from  the  Psychological  Laboratory  of  Harvard  University : 

I.  The  Relations  of  Intensity  to  Duration  of  Stimulation  in  our  Sensations  of 
Light:  James  E.  Lough,  484.  //.  Normal  Motor  Automatism :  Leon  M.  Solo- 
mons and  Gertrude  Stein,  492. 

On  the  Conditions  of  Fatigue  in  Reading:  Harold  Griffing  and  Shi 
HERD  Ivory  Franz,  513. 
/ij   Accuracy  of  Observation  and  of  Recollection  in  School  Children: 
Shepherd  Ivory  Franz  and  Henry  E.  Houston,  531. 
russion  and  Reports : 

Remarks  on  Professor  Lloyd  Morgan's  Method  in  Animal  Psychology :  Hiram 
M.  Stanley,  536.  Recognition :  Arthur  Allin,  Mary  Whiton  Calkins,  542. 
The  Community  of  Ideas  of  Men  and  Women:  Amy  Tanner,  548. 

Psychological  Literature : 

Recent  French  Works  {T.  Lachelier.A.  Fouillde,  T.  Halleux,  Ch.Mirallid,  Fr. 
Paulhan)  :  A.  Binet,  551.  Eucken's  Der  Kampf  um  einen  geistigen  Lebensin- 
halt:  A.  C.  Armstrong,  Jr.,  556.  Ethnology  and  Anthropology :  Livingston 
Farrand,  558.  L'Annde  psychologique :  H.  C.  Warren,  562.  Leuba's  The 
Psychology  of  Religious  Phenomena  :  Willkam  Romaine  Newbold,  569.  A 
New  Factor  in  Evolution:  J.  McKeen  Cattell,  571.  Vision:  C.  L.  Frank- 
lin, E.  B.  Delabarre,  573.  Localization  of  Touch :  Herbert  Nichols, 
577.  Memory:  H.  N.  Gardiner,  W.  G.  Smith,  578.  Synopsia:  Mary 
Whiton  Calkins,  581.  Psychical  Research:  J.  McKeen  Cattell,  582.  The 
Emotions:  H.N,  Gardiner,  583.  Epistemology :  C.  W.  Hodge,  H.  N.  Gar- 
diner, 584. 

New  Books.,  587;     Notes^  588. 


published  bi-monthly  by 
THE  MACMILLAN  COMPANY, 

66  FIFTH  AVENUE,  NEW  YORK;  AND  LONDON. 


\  1 1 


ON  THE  CONDITIONS    OF  FATIGUE   IN  READING.^ 

BY  HAROLD  GRIFFING  AND  SHEPHERD  IVORY  FRANZ. 

The  increasing  part  played  by  reading  in  the  life  of  civilized 
man  is  a  striking  characteristic  of  modern  culture.  In  fact,  the 
man  of  to-day  might  be  defined  as  a  reading  animal.  The  re- 
sult of  this  strain  upon  the  eye  has  been  the  wide  prevalence  of 
myopia,  astigmatism  and  kindred  disorders.  But  the  functions 
which  the  optic  mechanism  is  called  upon  to  perform  are  not 
abnormal ;  the  work  of  the  eye  differs  only  in  degree  from  that 
for  which  it  is  fitted.  If  the  eye  were  never  fatigued,  myopia 
would  be  rare. 

Yet  great  as  is  their  importance,  we  have  little  exact  knowl- 
edge of  the  conditions  of  minimum  visual  fatigue.  Cohn-,  Ja- 
vaP  and  Weber^  have  treated  the  subject  with  great  fulness, 
but  their  work  was  largely  theoretical.  CattelP  and  Sanford® 
have,  however,  investigated  the  subject  experimentally,  with 
special  reference  to  the  relative  legibility  of  letters. 

The  conditions  of  visual  fatigue  are  obviously  highly  com- 
plex. They  may  be  divided  into  two  classes.  On  the  one  hand, 
we  have  all  those  conditions  which  pertain  to  the  individual 
reader ;  for  example,  the  time  of  reading,  the  position  of  head 
and  eyes,  and  personal  peculiarities,  anatomical  and  physiologi- 
cal. Opposed  to  these  are  certain  purely  physical  conditions. 
Such  are  the  size  and  quality  of  the  type,  the  intensity  and 
quality  of  the  illumination,  the  color  and  quality  of  the  paper, 

^From  the  Psychological  Laboratory  of  Columbia  University.  Read  in 
condensed  form  before  the  International  Congress  of  Psychology,  Munich, 
August,  1896. 

2  Cohn,    T/ie  Hygiene  of  the  Rye  in  Schools,  Eng.  tr.,  London,  1886. 

■'Javal,  Annales  d'Oculiste,  79-82;  Revue  Scientifique,  1881. 

*  Weber,  Ueber  die  Augenuntersuchungen  iji  den  hoheren  Schulen  zii  Darm- 
stadt^ Referat  erstattet  d.  grossherz.  Ministerial,  Mdrz,  1881. 

^Cattell,  Philosophische  Studien,  III. 

^Sanford,  Americatt  Journal  of  Psychology,  I. 


i^  tt  .^  \)  {\ 


SH       HAROLD    GRIPPING   AND    SHEPHERD    I.  PRANZ. 

the  clearness  of  the  printing,  the  length  of  the  lines,  and  the 
spacing  between  the  letters  and  lines.  It  is  this  latter  group  of 
conditions  with  which  we  are  now  concerned. 

(i)  The  Size  of  the  Type. 

Weber  investigated  the  relation  of  the  size  of  type  to  legi- 
bility by  finding  the  maximum  rate  of  reading.  He  arrived  at 
the  paradoxical  result  that  although  the  rate  of  reading  de- 
creased for  very  small  type  it  also  decreased  when  the  height 
of  letters  was  over  2  mm^  By  determining  the  time  of  ex- 
posure required  for  perception  CattelP  studied  the  legibility  of 
small  Latin  letters  of  different  sizes,  .7,  i.i,  1.8,  2.5  and  5.8 
mm.  The  times  found  were  3,  1.4,  i.i,  .7  and  .6  for  one 
observer,  and  4,  1.7,  1.3,  .9  and  .7  for  the  other.  The  rela- 
tion is  approximately  expressed  by  an  hyperbolic  curve. 

The  investigations  of  Cattell  we  have  extended  and  supple- 
mented by  different  methods.  By  the  first  method,  which  we 
will  call  the  method  of  rapid  reading,  we  found  the  rates  at 
which  an  observer  could  read  printed  matter  in  large  and  small 
type.  Two  passages  of  the  Bible,  each  containing  622  words, 
were  used.  One  observer  read  one  passage  A,  in  large  type, 
and  another  passage  B,  in  small  type,  and  the  next  observer 
read  the  same  passages,  reversing  the  order  of  the  type,  read- 
ing A  in  small  type  and  B  in  large  type.  The  order  in  which 
the  experiments  were  made  was  also  reversed  for  alternate  ob- 
servers. The  time  was  taken  by  the  observer  with  a  stop 
watch,  but  recorded  without  his  knowing  the  result  by  one  of 
the  writers.  The  observers  were  mostly  students,  five  being 
familiar  with  experimental  psychology.  The  type  was  Roman, 
i.  e.^  the  ordinary  type  used  in  English  books.  The  large  type, 
of  which  we  here  give  examples,  was  Pica,  1.8  mm.  in  height, 
the  small,  Peari.  .9  mm.  in  height.  In  addition  to  these  experi- 
ments we  made  some  in  which  the  time  of  reading  was  con- 
stant, I  minute,  the  number  of  words  read  being  determined. 

Below  will  be  found  the  ratios  of  the  times  and  of  the  num- 
ber of  words  read. 

1  op.  cit. 

2  Not  given  bj  the  writer,  but  calculated  by  us  from  other  data  given. 


CONDITIONS    OF  FATIGUE   IN  READING.  515 

Table  I. — Relative  Times  for  Large  and  Small  Type. 


Observer. 

K 

A 

Di 

B 

S 

Fi 

F2 

F3 

G 

Hi 

Ha 

D, 

Av. 

Tx. 

•77 

1.04 

.82 

.61 

.90 

.88 

.72 

— 

1.08 

.96 

1. 01 

.92 

•qo 

Ws 

Wx. 

.88 

1. 00 

•91 

.42 

.65 

•94 

.80 

T 

=;,— =  ratio  of  time  required  to  read  large  type  to  that  required  to  read  small  type. 

Ws  _  ratio  of  number  of  words  read  in  one  minute  in  small  type  to  the  number 
Wl       read  in  large  type. 

In  a  few  additional  experiments  the  observers  read  at  their 

T 
natural  rates.    The  resulting  ratios  ^p-  for  4  observers  were  .87, 

^  s 

1. 00,  .86  and  .81,  the  average  .89,  being  the  same  practically 
as  that  obtained  by  the  other  method. 

Thus  it  takes  on  the  average  about  -^-^  as  much  time  to  read 
large  type,  1.8  mm.,  as  to  read  small  type,  .9  mm.  The  dif- 
ference in  legibility  would  probably  be  much  greater  were 
it  not  that  when  the  small  type  is  read  more  words  can  be 
seen  simultaneously.  In  this  way  we  may  explain  Weber's 
paradoxical  result.  As  the  size  of  the  letters  increases  be- 
yond a  certain  limit  the  rate  of  reading  will  necessarily  de- 
crease ;  but  this  does  not  involve  an  increase  of  fatigue,  as 
Weber  assumed. 

By  a  second  method  we  found  the  relative  number  of  words 
seen  when  exposed  for  J-g-  sec.  by  Cattell's  gravity  chronometer.^ 
Phrases  of  three  and  four  words  were  pasted  on  white  strips  of 
cardboard  and  were  shown  for  the  time  desired  by  a  falling 
screen.  The  greater  part  of  the  screen  was  hidden  from  the 
view  of  the  observer  by  a  black  sheet  of  paper  with  an  opening 
where  the  letters  were  to  appear.  The  phrases  were  cut  from 
the  books  mentioned,  the  letters  being  1.8  and  .9  mm.  high. 
None  of  the  words  were  of  more  that  two  syllables.  The  same 
phrases  were  used  for  large  and  small  type.  There  were  54 
phrases  of  3  words  and  54  of  4  words,  half  in  large  type  and 
half  in  small.     Thus  there  were  216  -(-  162  words  in  all. 

^  For  description  of  the  instrument  see  op.  cit. 


5l6        HAROLD    GRIPPING  AND    SHEPHERD   I.  FRANZ. 

The  experiment  was  conducted  as  follows  :  The  observer 
took  his  seat  in  a  comfortable  chair  opposite  the  instrument  and 
placed  his  chin  upon  a  rest  suitably  adjusted,  so  that  his  eyes 
were  slightly  above  the  level  of  the  letters  exposed,  and  30  cm. 
distant  from  them.  The  experimenter  (one  of  the  writers) 
stood  behind  the  instrument  so  as  to  adjust  the  cards  with  the 
phrases.  When  the  card  was  placed  the  observer  fixated  a 
gray  cross  on  the  black  background  of  the  movable  screen 
directly  in  front  of  the  letters,  and  let  the  screen  fall  by  break- 
ing the  current  with  a  Morse  key.  He  then  wrote  down  what  he 
thought  he  had  seen.  A  dozen  or  more  practice  trials  were 
made  before  beginning  the  experiments  proper.  The  observer 
was,  of  course,  ignorant  of  the  phrases  that  were  to  be  given. 
Care  was  taken  not  to  have  a  phrase  already  given  in  one  type 
repeated  immediately  in  another.  Of  eleven  observers  six  com- 
pleted only  half  of  the  series.  We  give  below  the  results  for 
the  different  observers. 


Table  II. — Percentages  of  Words  Seen;   Large  and 

Small  Type. 


Three-Word  Phrases. 

Four-Word  Phrases. 

Observer, 

S. 

L. 

s         1 

S. 

L. 

l=- 

H. 

.22 

•56 

•39 

13 

•44 

•29 

C. 

.46 

•75 

.61 

59 

•75 

•79 

T.  G. 

.29 

•75 

•39 

23 

.60 

.38 

I.  F. 

.60 

•95 

•63 

80 

.88 

.88 

H.  G. 

.46 

.81 

.56 

66 

.96 

.69 

P. 

.46 

•91 

•50 

45 

•85 

•53 

L. 

.10 

•  54 

.18 

iS 

•32 

•56 

R.  G. 

.76 

•79 

.96 

48 

.68 

.70 

S. 

.12 

.47 

•25 

12 

•39 

•31 

A. 

.68 

•  78 

.87 

55 

.69 

•79 

S.  F. 

•43 

•85 

•51 

59 

.81 

•73 

Average 

•53 

1        .60 

Vertical  columns  S  and  L  give  percentages  of  words  seen  for  small  and 
large  type  (.9  and  1.8  mm.  high). 

Vertical  column  =-  give  ratios   in  per   cent,  or  the  relative  legibility  X  of 

small  and  large  type. 

With  the  observers  whose  initials  are  given  in  block  tj'pe  the  full  set  of  ex- 
periments (108)  were  made,  only  39  being  made  on  the  others. 

In  taking  the  average  the  values  of  A  for  these  five  might  be  weighted.  This 
would  change  the  averages  somewhat. 


CONDITIONS    OF  FATIGUE   IN  READING.  5^7 

From  the  above  table  we  see  that  on  the  average  but  little 
more  than  one  half  as  many  words  were  seen  in  small  type  as 
in  large  type.  Individual  variations  are  great,  but  these  vari- 
ations are  probably  not  due  to  an  appreciable  extent  to  individ- 
ual differences  in  the  relative  legibility  of  large  and  small  type. 
For  good  observers  the  same  difference  in  legibility  would  give 
different  values  of  I. 

This  theoretical  conclusion  is  verified  by  the  experiments. 
By  arranging  the  observers  in  two  groups  according  to  the  per- 
centages seen,  the  values  of  I  is  for  the  better  observers  in  all 
cases  lower  than  that  of  any  of  the  four  poorest  observers. 

A  few  experiments  were  made  with  21  two-word  phrases 
printed  in  very  large    type   (4+   mm).     The  percentages    of^ 
words  seen    correctly   by   three    observers,    together  with   the 
averages  of  the  same  observers  for  1.8  mm.  type  as  found  from 
the  table  above  given  are  as  follows  : 

Large  Very  large 
P.                                    .88  ".93 

L.  .43  -64 

S.  .43  -7° 

Thus  the  legibility  as  shown  by  this  method  appears  to  in- 
crease regularly  with  the  size.  But  since  the  number  of  words 
brought  within  the  field  of  distinct  vision  decrease  with  the  size, 
the  relation  is  quite  complex. 

A  few  phrases  (15)  of  two  words  each  were  used  with  the 
others.  The  percentages  for  two,  three  and  four-word  combi- 
nations were  found  to  vary  but  little  with  the  number  of  words. 

From  the  table  it  will  be  seen  that  the  values  of  l  were  about 
the  same  for  phrases  of  three  words  as  for  those  of  four  words, 
the  averages  for  phrases  of  2,  3  and  4  words  in  small  type  be- 
ing .42,  .41  and  .43. 

In  the  above  experiments  the  paper  was  not  exactly  the 
same  for  large  and  small  type,  being  slightly  grayish  for  the 
small  type  and  of  a  more  yellowish  tint  for  the  large.  To 
eliminate  this  source  of  error,  phrases  of  four  words  in  large 
and  small  type  were  printed  on  the  same  white  paper.  From 
200  experiments  (800  words),  100  on  S.  F.  and  lOO  on  H.,  we 
found  the  following  percentages  of  words  seen : 


5l8        HAROLD    GRIPPING   AND    SHEPHERD   I.    PRANZ. 


H. 

S.  F. 


.12 
•83 


.90 


L 

•37 
.92 


=  A 


The  values  of  A  correspond  quite  closely  with  those  pre- 
viously found  for  the  same  observers,  .88  for  F.  and  .29  for  H. 

A  modification  of  the  preceding  method  was  used  b}^  deter- 
mining the  time  words  composed  of  letters  of  different  sizes  had 
to  be  exposed  in  order  to  be  seen.  This  we  will  call  the  time 
of  exposure  method.  The  same  apparatus  was  used  as  before, 
the  time  of  exposure  varying  with  the  extent  of  opening  of  the 
screen.  This  time  can  be  determined  to  about  .150",  a  being 
.001  sec.  The  words  were  of  not  less  than  5  letters,  nor  over  2 
syllables,  on  white  paper.  The  type,  as  here  shown,  was 
six  point  and  'eleven  point,'  .8  and  1.6  mm.  high.  On  ac- 
count of  the  preliminary  practice  necessary  there  were  but  three 
observers,  two  being  the  writers.  The  experiments  were  con- 
ducted in  the  same  general  way  as  those  just  described.  The 
experimenter  tried  first  very  small  times,  increasing  the  time 
until  the  stimulus  was  perceived  approximately  50%  of  the 
time.  Then  other  words  were  shown  which  the  observer  had 
not  seen.  As  the  percentage  seen  tends  to  increase  very 
rapidly  from  o  to  100  (theoretically  99+  ),  it  was  generally 
easy  to  determine  at  one  sitting  the  time  required  either  directly 
or  by  estimation  from  the  percentage  seen.  The  times  of  ex- 
posure found  thus  are  now  given  in  thousandths  of  a  second. 

Table  III. — Times  of  Exposure  for  Different  Sizes  of 
Type  to  be  Seen. 


L 

Observer. 

L. 

S. 

S  =  ^ 

G. 

1.6 

1-9 

.84 

f 

I.I 

1-5 

■73 

" 

1-3 

1-7 

.76 

H 

2.0 

2.8 

•71 

1  i 

1.6 

2.5 

.64 

AV. 

.73 

L  and  S  denote  the  times  of  exposure  necessary  for  large  and  small  type  re- 
spectively, .8  and  1.6  mm. 

^  or  /I  is  the  relative  legibilty  measured  by  this  method. 

The  two  values  of  L  and  S  for  F  and  H    are  for  different  days.     The  time 
of  exposure  seems  to  vary  in  the  same  individual. 


CONDITIONS   OF  FATIGUE  IN  READING.  519 

From  the  above  results  it  appears  that  the  large  type,  1.6 
mm.,  requires  about  ^  as  great  a  time  of  exposure  as  the  small 
type  of  half  the  height,  .8  mm. 

In  the  last  two  sets  of  experiments  a  few  observations  were 
made,  which  though  not  bearing  on  the  special  problems  under 
investigation  are  yet  of  psychological  interest.  Observers  gen- 
erally failed  to  see  any  of  the  letters  making  up  a  word  when 
they  failed  to  perceive  the  whole  word.  There  were,  however, 
individual  differences,  some  persons  often  seeing  one  or  two 
letters  only.  At  times  an  observer  saw  combinations  without 
sense,  though  he  knew  such  combinations  were  not  given.  In 
the  time-of-exposure  experiments  the  observer  was  at  times  con- 
scious of  perceiving  letters  without  knowing  what  they  were. 
Occasionally  the  observer  had  an  impression  that  a  given  word 
was  present,  when  the  letters  had  not  appeared  distinctly.  More 
often  some  letters  were  distinct,  and  he  guessed  the  word,  or 
else  the  whole  word  was  distinct.  One  of  the  writers  had  a 
marked  tendency  to  see  again  what  had  been  given  before,  even 
when  he  knew  that  the  word  was  not  repeated.  One  of  the  ob- 
servers, H.,  seemed  to  be  an  exception  to  the  rule  that  one  sees 
all  letters  exposed  or  none  at  all  except  within  very  small  range 
of  time.  Some  days  it  was  very  difficult  to  find  the  time  re- 
quired for  this  reason.  But  perhaps  the  most  important  phe- 
nomenon observed  was  the  illusory  perception  of  a  word,  the 
letters  appearing  distinct  when  not  present.  This  has  been  al- 
ready noted  by  Cattell  and  also  by  Miinsterberg.  The  theo- 
retical importance  of  this  lies  in  the  support  which  it  gives  to 
the  hallucination  theory  of  perception.  The  representative  pro- 
cesses in  perception  seem  to  attain  to  the  sensory  vividness  of 
true  hallucinations.  This  does  not,  however,  appear  to  take 
place  in  every  instance,  for  F.  seemed  at  times  to  see  some  of 
the  letters  and  to  infer  by  ordinary  processes  of  association  that 
a  certain  word  was  present. 

To  obtain  more  extended  results  and  confirm  those  obtained 
by  Cattell,  by  the  time-of-exposure  method,  we  determined  the 
intensity  of  illumination  necessary  for  the  reading  of  letters  of 
different  sizes.  The  letters  were  printed  in  the  simplest  kind 
of    type,    commonly   called   Block.      Two    cards    were,    how- 


520       HAROLD    GRIFFING  AND    SHEPHERD   I.  FRANZ. 

ever,  covered  with  words  in  Roman  type,  .8  and  1.6  mm.  in 
height. 

The  observer  sat  in  front  of  a  stand  from  a  projecting  piece 
of  which  was  suspended  a  small  pendulum  making  a  vibration 
in  Yi  sec.  The  pendulum  swung  in  front  of  a  screen  having 
an  opening  where  the  letters  to  be  seen  appeared.  The  letters 
were,  of  course,  shown  ^^  sec.  The  letters  were  posted  on  card- 
board strips  and  these  were  placed  in  slits.  The  paper  was  the 
same  for  the  different  sizes,  pure  white.  The  slits  were  ar- 
ranged so  that  the  length  of  the  cardboard  exposed  was  either 
15  or  3  mm.,  according  to  the  size  of  the  letters.  For  the  two 
largest  sizes,  and  also  for  the  cards  on  which  the  words  in  Ro- 
man type  were  shown,  the  large  area  was  used.  The  object 
was  to  show  only  one  or  two  letters  at  a  time,  except  when  the 
Roman  type  was  used,  when  a  larger  number  was  seen.  A 
black  screen  in  front  of  the  pendulum  with  the  necessary  open- 
ing served  to  prevent  distraction  of  the  observer  by  the  move- 
ment of  the  pendulum. 

The  observer's  eyes  were  kept  at  a  constant  distance  (30  cm.) 
from  the  stimulus  by  means  of  a  chin  rest.  The  light  was  that 
of  a  hooded  petroleum  lamp  found  to  be  fairly  constant,  shin- 
ing through  a  square  of  ground  glass  5x5  mm.  The  light 
emitted  was  approximately  .02  candle  power.  The  lamp  was 
in  a  movable  box  sliding  on  wheels  in  iron  grooves.  Precau- 
tions were  taken  to  avoid  errors  from  reflected  or  diffused  light. 
The  letters  used  were  in  combinations  of  one  to  four  words  in 
one  horizontal  line.  They  were  taken  from  a  printer's  sample 
book.  The  median  plane  of  the  observer  was  approximately 
perpendicular  to  the  plane  of  the  cardboard  to  be  seen,  and  the 
lamp  could  be  moved  only  in  a  straight  line,  making  an  angle 
of  45°  with  the  plane  of  the  cardboard. 

With  this  apparatus  after  the  observer  had  remained  in  the 
dark  room  long  enough  to  avoid  errors  from  adaptation  (20  to 
30  min.),  the  experiment  was  made  as  follows:  A  card  with 
letters  to  be  exposed  was  placed  in  the  slit  by  the  experimenter 
(one  of  the  writers).  The  observer  pushed  back  the  pendulum 
to  a  fixed  support  with  his  hand,  fixated  a  pencil  cross  on  the 
cardboard  piece  fastened  to  the  pendulum  directly  in  front  of 


CONDITIONS    OF  FATIGUE   IN  READING. 


121 


the  letters  to  be  seen,  and  then  let  the  pendulum  swing  forward, 
observing  the  letters  as  they  were  shown.  As  the  pendulum 
swung  back  it  was  caught  by  the  observer  with  the  left  hand 
and  fixed  with  a  catch.  He  then  moved  the  lamp  nearer  with 
the  right  hand.  At  first  this  was  done  by  the  experimenter, 
but  with  less  convenience  and  economy  of  time.  This  was  re- 
peated until  the  observer  was  quite  certain  he  could  perceive  the 
letters  correctly  when  exposed  but  once.  The  distance  of  the 
light  from  the  letters  was  then  read  off  on  a  scale.  The  square 
of  the  reciprocal  of  this  distance  represents  the  relative  intensity 
of  the  illumination.  The  readings  were,  of  course,  taken  by  the 
experimenter.  For  this  purpose  we  used  the  light  from  a  small 
candle  inside  a  blackened  box  shining  through  a  cylindrical 
tube.  Two  or  three  determinations  were  generally  made  at  one 
sitting  for  each  of  the  variables  under  investigation,  including 
several  in  addition  to  the  type.  Variations  in  the  results  made 
it  necessary  to  average  the  records  of  some  days  separately,  as 
given  in  the  second  horizontal  columns  for  F  and  H. 

We  give  below  the  average  values  of  T,  the  illumination 
threshold^  for  reading  in  terms  of  one  candle-meter  (CM. ) ,  or  the 
light  of  a  standard  candle  at  a  perpendicular  distance  of  one  meter. 

Table  IV. — Illumination  Thresholds  for  Different 
Sizes  of  Type. 


Observer. 

N 

H  =  .9 

H=  1.6 

H=3.li 

1 

H  =6.0 

h=  . 

h  =  1.6 

Av 

MV 

Av 

MV 

Av 

.042 

MV 

Av 

MV 
.001 

Av 
.36 

MV 

.04 

Av 

MV 

G 

lO 

27 

.02 

.12 

.01 

.003 

.014 

.14 

.01 

F 

6 

.24 

.02 

.08 

.01 

.028 

.007 

.010 

.001 

.22 

.02 

.12 

.02 

(< 

3 

•17 

•03 

•045 

.004 

.018 

.002 

.008 

.001 

•13 

.01 

.05 

.00 

H 

5 

.077 

.014 

•035 

.007 

.014 

.001 

.003 

.000 

•19 

.01 

.07 

.00 

3 

•19 

.02 

.09 

.003 

•043 

.003 

.009 

.001 

•35 

•03 

•13 

.02 

H=height  of  Gothic  letters  in  mm. 

h=height  of  Roman  letters  in  mm. 

N=  number  of  determinations  upon  which  average  is  based, 

Av=  average. 

MV=mean  variation. 

/Icos0 


^  Calculated  bj  the  formula  T= 


d2 


where  A  is  the  candle  power  of  the 


light,  d  the  distance  of  the  light  from  the  object,  and  fl  the  angle  made  by  the 
normal  to  the  surface. 


522        HAROLD    GRIFFING   AND    SHEPHERD    I.  FRANZ. 

A  graphical  representation  of  the  results  is  shown  in  the  ac- 
companying figure.  The  ordinates  give  the  intensity  of  illumi- 
nation in  candle-meters,  and  the  abscissas  the  height  of  the 
letters  in  tenths  of  millimeters. 


lb 


31 
Fig.  I. 


hO 


The  curves  resemble  rectangular  hyperbolas,  the  values  of 
the  variables  corresponding  roughly  to  the  equation, 

(s— k)  i=ki, 

k  and  kj,  being  constants  depending  upon  the  individual.  As- 
suming such  an  equation  w^e  may  infer  that  after  the  size  of  the 
type  has  reached  a  certain  limit  the  increase  of  size  is  in  direct 
proportion  to  the  decrease  of  illumination.  The  fatigue  coeffi- 
cient increases  slowly  until  the  size  of  the  type  decreases  to 
about  2-3  mm.,  after  which  its  increase  is  more  and  more  rapid. 
The  lowest  limit  to  the  size  of  type  in  common  use  should  be 
1.5  mm.  The  same  conclusion  may  be  drawn  from  the  experi- 
ments of  Cattell  already  mentioned. 

(2)   Thk  Quality  of  the  Type. 

On  theoretical  grounds  it  may  be  assumed  that  the  legibility 
of  letters  decreases  with  increasing  complexity  of  structure. 
From  this  point  of  view  German  type  is  open  to  serious  criti- 


CONDITIONS    OF    FATIGUE  IN  READING.  5^3 

cism,  and  even  our  Roman  type  might  evidently  be  much  im- 
proved. Some  of  our  letters  have  unnecessary  features  and 
they  are  as  a  rule  much  more  complex  in  structure  than  those 
printed  in  the  so-called  Block  type.  Many  letters,  such  as  c 
and  e,  are  easily  confused,  and  there  are  decided  differences  ot 
legibility.  These  differences  are,  indeed,  slight  and  difficult  to 
determine.  By  finding  the  percentage  of  times  each  letter  was 
seen  when  exposed  for  i<t,  more  or  less,  CattelP  found  the  order 
of  legibility  of  the  small  letters  to  be  :  d  k  m  q  h  b  p  w  u  1  j 
tvzrof  naxyeigcs.  There  seemed  to  be,  however, 
individual  differences.  Sanford^  by  a  different  method  found 
a  somewhat  different  order  of  legibility. 

In  the  writers'  experiments,  which  were  made  only  by  the 
most  delicate  method,  that  of  the  illumination  threshold,  the 
following  styles  of  type  were  used:  Roman  (small  letters), 
that  used  universally  in  England,  America  and  southern 
Europe  for  books  and  newspapers ;  German,  or  that  used  in 
Germany  ;  Block,  in  which  the  letters  are  of  uniform  thickness 
and  of  the  simplest  shape,  much  Hke  Roman  capitals.  Two 
styles  of  Block  were  used,  as  here  shown  ;  in  one  the  letters 
being  quite  thick,  .5  mm.,  whereas  in  the  other  they  were  .15 
mm.  Besides  the  ordinary  Roman  letters  there  were  two  other 
sets  in  semi-Roman  type  ;  one,  Roman  II.,  having  very  thick  and 
very  thin  lines,  .05  to  .5  mm.  ;  the  other,  Roman  III.,  being  some- 
what like  the  plainer  Block  and  of  uniform  thickness,  about  .2 
mm.  The  size  of  the  letters  was  practically  constant  for  the 
different  groups,  1.5  mm.  in  height,  there  being,  however, 
slight  variations,  to  .1  mm.  in  the  individual  letters. 

We  give  now  the  results  in  tabular  form.  The  figures  mean 
the  same  as  in  Table  IV.  The  results  for  F.  and  H.  are  given 
in  2  columns  on  account  of  a  variation  in  sensibility  which 
made  it  necessary  to  average  the  results  of  the  earlier  experi- 
ments separately. 

1  op.  cit. 


524        HAROLD    GRIPPING    AND    SHEPHERD   I.  FRANZ. 

Table  V. — Illumination   Threshold  for  Different 
Kinds  of  Type. 


Roman 
I. 

Roman 
II. 

Roman 
III. 

German 

Block 

THIN. 

Block 

THICK. 

Observer. 

N 

AV 

MV 

AV 

MV 

AV 

MV 
.02 

AV 

MV 

AV 

MV 

AV 

.09 

MV 

G. 

10 

.22 

.02 

.12 

.02 

.18 

.21 

•03 

.20 

.02 

.01 

F. 

6 

.14 

.01 

.12 

.02 

•13 

.01 

•15 

.02 

.13 

.01 

.07 

.01 

" 

S 

.11 

.01 

.06 

.001 

.08 

.00 

.12 

.01 

.10 

.00 

.06 

.00 

H. 

.5 

.10 

.01 

.06 

.01 

.12 

.02 

.12 

.02 

.10 

.02 

.04 

.00 

" 

3 

.22 

•03 

•  15 

•03 

.22 

.02 

.24 

.02 

.22 

.01 

.10 

.00 

From  the  above  table  we  may  calculate  the  relative  legibility 
I  of  the  different  styles  of  type  ;  I  of  course  being  the  reciprocal 
of  the  illumination  threshhold  given  above. 

The  values  of  1  are  now  given. 


Table  VI. — Relative  Legibility  of  Type,  that  of  Ro- 
man being  I. 


Observer. 

Roman  II. 

Roman  III. 

German. 

Block 

THIN. 

Block 

THICK. 

G. 

1.8 

1.2 

I.O 

I.I 

2.4 

F. 

I.I 

I.I 

•9 

I.I 

2.0 

" 

1.8 

1.4 

•9 

i.i 

1.8 

H. 

1.7 

.8 

.8 

1.0 

2.5 

" 

1-5 

I.O 

•9 

I.O 

2.2 

Av 

1.6 

I.I 

•9 

I.I 

2.2 

From  the  above  we  see  that,  contrary  to  our  expectation,  the 
difference  in  legibility  between  Roman  and  German  type  is  rela- 
tively slight.  Thin  hair  lines,  if  accompanied  by  thick  lines,  do 
not  seem  to  diminish  the  legibility,  Roman  II.  requiring  nearly 
half  as  much  light  as  Roman  I.  The  complexity  of  the  letters, 
within  the  limits  here  studied,  does  not  seem  to  have  decided 
effect  on  the  legibility,  for  the  value  of  /  for  thin  Block  is  about 
the  same  as  for  Roman.  The  greater  legibility  of  Block  type 
is  due  almost  entirely  to  the  thickness  of  the  letters,  as  shown 
by  these  experiments.  On  the  other  hand,  if  a  part  of  the  letter 
is  thick  it  is  quite  legible,  even  though  thin  hair  lines  are  fre- 
quent.    It  is,  however,  probable  that  type  such  as  Roman  II.  is 


CONDITIONS   OF  FATIGUE  IN  READING.  525 

more  fatiguing  than  the  results  indicate.  It  may  be  possible 
for  the  mind  to  perceive  certain  objects  with  fatigue  when  other 
objects  are  either  perceived  without  appreciable  fatigue,  or  not 
perceived  at  all. 

(3)   The  Distance  Between  the  Letters  and  Lines. 

The  horizontal  distance  between  the  letters  has  been  said  by 
Javal  to  be  of  great  importance.  Certainly  a  word  printed  so 
that  these  distances  is  increased  .8  to  1.3  mm.  appears  to  be 
much  more  distinct.  The  effect  of  an  increase  in  this  spacing 
is  here  shown.  But  twelve  experiments  on  three  observers  by 
the  illumination  threshold  method  gave  negative  results.  We 
must  conclude  then  that  the  spacing  commonly  used  is  quite 
sufficient.  Greater  spacing.would,  of  course,  be  more  expensive, 
and  the  decrease  of  fatigue  not  as  great  as  might  be  brought 
about  in  other  ways. 

As  regards  the  vertical  space  between  the  lines,  technically 
called  « leading,'  a  slight  effect  on  legibility  was  found  when 
the  distance  with  Pearl  type,  .8  mm.  high,  was  increased  from 
.8  to  1.3  mm.  The  illumination  threshold  method  was  used, 
and  the  experiments  carried  on  simultaneously  with  the  preced- 
ing.    The  following  results  were  obtained  : 


Table  VIL — Illumination  Threshold  for  Type  Leaded 
AND  NOT  Leaded. 


.8  mm. 

1-3 

mm. 

/. 

Av 

MV 

Av 

MV 

G. 

.40 

.03 

.36 

.04 

.90 

F. 

•25 

.01 

.22 

.02 

.88 

" 

.12 

.01 

.12 

.00 

1. 00 

H 

.12 

.01 

.09 

.01 

•75 

•35 

.02 

.27 

.01 

•77 

Av 

.86 

Thus  the  average  relative  legibility   of  unleaded  type   to 
leaded  type,  as  measured  in  this  way,  is  about  .9. 


526     harold  gripping  and  shepherd  i.  pranz. 

(4)  The  Intensity  of  Illumination. 
Although  the  variation  in  the  intensity  of  diffused  daylight 
in  a  well  lighted  room  is  known  to  be  very  great,  even  when  the 
other  conditions  such  as  time  and  place  are  constant,  being 
roughly  from  50  to  1500  candle-meters,^  no  results  were  ob- 
tained for  variations  in  legibility  due  to  this  variation  by  the  two 
gravity  chronometer  methods  and  the  method  of  rapid  reading. 
The  problem  is,  however,  difficult  to  investigate  in  this  way  by 
reason  of  the  marked  daily  variations  in  individual  sensibility. 
It  was  necessary,  therefore,  to  use  artificial  light  of  low  intensity. 
The  relation  of  the  intensity  and  the  legibility  under  these  con- 
ditions has  already  been  studied  by  Cattell  by  the  time  of  expo- 
sure method.  Using  the  light  of  a  petroleum  lamp  (about  10 
candle  power)  18  cm.  distant  at  an  angle  of  55°  as  the  unit  of 
illumination,  u  e.,  about  260  c.  m.,  the  times  of  exposure  for 
the  intensities  i,  ^,  Jg'  eV'  2i6'  were  i.^cr,  1.7,  2.5,  6.  and  20. 
In  order  to  supplement  the  work  of  Cattell  we  determined 
the  maximum  rate  of  reading  for  different  intensities  of  illumina- 
tion. It  had  already  been  found  by  Weber^  that  for  low  intensi- 
ties the  rate  of  reading  varies  with  the  illumination.  Our  ex- 
periments were  made  in  the  following  manner  :  The  book  to  be 
read,  the  Pearl  type  Bible  already  mentioned,  was  fastened  on 
a  wooden  stand  so  as  to  be  in  front  of  the  observer,  and  making 
an  angle  of  45°  with  the  rays  of  light.  The  light  used  was  a 
standard  candle  placed  inside  a  blackened  box.  The  conditions 
were  such  that  the  light  came  from  behind  the  observer  and  to 
his  left.  The  observer  read  one  column  as  fast  as  possible,  re- 
cording the  time  with  the  stop  watch.  With  the  lowest  inten- 
sity, however,  on  account  of  fatigue,  but  half  a  column 
was  read,  the  time  being  doubled  for  the  whole  column. 
The  observer,  it  should  be  added,  remained  in  a  dark  room 
long  enough  to  avoid  errors  from  adaptation.  The  experi- 
ments were  made  on  one  day  by  each  observer.  Below  are 
given  the  results  in  seconds  for  the  different  distances  in  meters 
and  the  relative  intensities  in  candle-meters. 


1  Cohn,  0^.  cit. 

2  Weber,  op.  cit. 


CONDITIONS    OF  FATIGUE   IN  READING.  527 

Table  VIII. — Time  of  Reading  at  Different  Intensities. 


Observer. 

Day- 

X M. 

K  M. 

I  M. 

iK  M. 

2  M. 

light. 

II. 2   C.  M. 

2.8  C.  M. 

.7   C.  M. 

.35  C.  M. 

.17  C.  M. 

HG 

K 

35 

36 

36 

46 

63 

no 

45 

44 

39 

53 

83 

120 

F 
G 

S 

47 

51 

52 

69 

100 

170 

47 

49 

59 

72 

130 

— 

29 

29 

35 

48 

— 

In  these  experiments  the  rate  of  reading  does  not  appear  to 
be  appreciably  affected  by  a  decrease  of  illumination  within 
a  very  wide  range,  the  intensity  of  good  daylight  being  about 
500  times  as  bright  as  the  lowest  intensity  here  used,  with  which 
the  rate  of  reading  was  not  appreciably  increased.  We  con- 
clude then  that  within  wide  limits  such  as  those  of  ordinary 
daylight  variation  in  the  intensity  of  illumination  is  not  attended 
by  great  fatigue.  But  when  the  illumination  decreases  to  a  cer- 
tain point,  not  far  from  3  CM.,  the  fatigue  becomes  excessive. 
This  is  shown  by  the  fact  that  very  slight  differences  in  the  rate 
of  reading  are  caused  by  conditions  of  great  fatigue,  an  increase 
of  about  \  in  the  time  of  reading  corresponding  to  decrease  in 
the  illumination  threshold  of  70  per  cent. 

The  above  experiments  correspond  quite  well  with  those  of 
Cattell  by  the  time-of-exposure  method.  His  results  show  that 
the  fatigue  coefficient  increases  very  rapidly  as  the  illumination  de- 
creases below  approximately  4  C.  M.  His  experiments  also 
show  that  the  fatigue  coefficient  is  appreciably  greater  for  the 
lamp  hght,  about  250  C.  M.,than  for  daylight,  and  that  it  in- 
creases as  the  illumination  is  further  decreased. 

(5)  The  Quality  of  the  Illumination. 

The  use  of  artificial  light  has  long  been  recognized  as  an 
important  cause  of  visual  fatigue.  This  fatigue  may  be  partly 
ascribed  to  the  conditions  of  intensity,  the  light  of  a  good 
petroleum  lamp  at  convenient  reading  distance  being  less  than 
that  of  good  daylight.  We,  therefore,  tested  the  effect  of  artifi- 
cial light  of  high  intensity  by  using  the  light  from  an  incan- 
descent Welsbach  gas  burner  giving  clear,  white  light,  35  can- 
dle power  at  25  C.  M.  and  45°,  about  400  C.  M.     The  times  of 


528        HAROLD    GRIPPING   AND    SHEPHERD   I.  PRANZ. 

exposure  required  for  perception  by  the  writers  were  found  to 
be  as  given  below. 

Small  Type.  Large  Type. 

Welsbach.     Daylight.  Welsbach.     Daylight. 

G  1.8  1.9  —  — 

These  values  are  thus  smaller,  rather  than  larger,  than  those 
already  found  for  daylight.  We  must  suppose  the  decrease  in 
time  to  be  due  to  daily  variations.  The  above  measurements 
were  made  on  one  day,  and  the  perceptive  and  retinal  processes 
of  F  were  more  than  usually  delicate.  The  smallest  time  found 
.8<T,  is  about  as  small  as  any  found  by  Cattell  in  all  his  experi- 
ments. It  is  evident,  therefore,  that  with  sufficient  intensity  of 
white  artificial  light  the  legibility  of  printed  matter  may  be  as 
great  as  in  good  daylight. 

Gas  light  and  lamplight  have,  in  addition  to  their  frequent 
unsteadiness,  the  disadvantage  of  a  yellow  color.  Since,  as 
will  be  seen  later,  yellow  paper  is  unfavorable  for  reading,  yel- 
low light  causing  the  paper  to  appear  yellow  must  also  be  a 
source  of  fatigue. 

(6)  The  Quality  of  the  Paper. 

If  the  paper  used  reflects  very  little  light  and  is  of  such  a 
quality  that  letters  can  be  well  printed,  the  exact  hue  is  probably 
of  little  importance,  provided  a  large  quantity  of  light  be  dif- 
fused. But  if  the  absorption  be  so  great  that  the  paper  appears 
grayish,  letters  printed  on  it  will  not  be  so  legible  by  reason  of 
the  lessening  of  the  contrast  between  the  letters  and  the  back- 
ground. 

In  experiments  made  by  the  different  methods  already  de- 
scribed we  used  non-reflecting  clear  white  paper  and  gray  paper, 
technically  called  news-paper,  the  same  as  that  used  by  many 
newspapers,  only  slightly  darker.  By  the  color-wheel  method 
it  was  found  that  the  white  paper  used  had  to  have  30  per  cent, 
black  mixed  with  it  to  give  a  gray  corresponding  to  this.  Its  rel- 
ative luminosity  was  therefore  about  .70.  Specimens  of  red  and 
yellow  paper  were  also  used,  the  red  corresponding  to  the  spec- 


CONDITIONS   OF  FATIGUE   IN  READING. 


529 


trum  color  just  to  the  left  of  Frauenhofer's  line  C,  and  the  yel- 
low that  to  the  right  of  line  D  {\  of  the  distance  to  line  E) . 

Experiments  by  the  method  of  the  percentage  of  words  seen 
on  one  observer  with  ii-point  type  gave  negative  results,  the 
percentages  of  words  seen  out  of  150  being  32  per  cent,  and 
and  31  per  cent.,  the  same  for  white  paper  as  for  the  newspaper. 
Of  small  type  words,  6-point,  given  at  the  same  time,  the  same 
observer,  H.,  saw  but  12  per  cent. 

By  the  time-of-exposure  method,  however,  different  results 
were  obtained.     Below  are  the  times  found  for  two  observers. 


G. 

F. 


White. 
2.8 
1.2 


News. 
4.0 
1-7 


Yellow. 
4.0 
2.5 


Red. 


4.0 


Thus  the  time  of  exposure  is  considerably  longer  for  gray 
tinted  paper,  as  well  as  red  and  yellow  paper,  than  for  white. 
The  explanation  of  the  greater  legibility  of  the  letters  on  white 
paper  over  those  on  the  red  and  yellow  is  the  same  as  for  the 
gray.  Color  quality  is  not  independent  of  intensity,  white 
being  essentially  brighter  than  yellow,  which  in  turn  is  brighter 
than  red. 

The  illumination  method  was  also  applied  to  the  study  of 
the  fatigue  effect  of  white  paper  and  gray  newspaper.  The 
letters  were  not  read  independently  in  these  experiments,  but  in 
words.  Upon  the  paper  exposed  were  10  to  12  words  in  3 
lines. 

The  values  of  the  illumination  threshold  were  as  follows  : 


Table    IX. — Illumination    Thresholds    for  White    and 

Gray  Paper. 


Observer. 

G 

F 

Fi 

H 

Hx 

Av 

MV 

Av 

MV 

Av 

MV 

Av 

MV     Av 

MV 

W  =  White 

N  =  News 

N-^ 

.10 
.20 

•50 

.01 

.02 

.10 
.16 

.62 

.01 
.02 

.06 
.08 

•75 

.01 
.01 

.04 

.07 

•57 

.00 
.00 

.10 
•23 

•43 

.02 
.01 

According  to  these    results  the  gray  tinted  newspaper  re- 
quired about  twice  as  much  illumination  as  the  white.     This  is 


530       HAROLD    GRIPPING  AND    SHEPHERD   I.  PRANZ. 

somewhat  more  than  might  be  expected  from  the  relative  ab- 
sorption powers  of  the  papers,  but  the  quality  of  the  printing 
varies  with  the  paper,  not  being  quite  so  clear  on  the  news- 
paper. 

Summarizing  briefly  our  results  we  conclude  that  the  size  of 
type  is  the  all  important  condition  of  visual  fatigue.  No  type 
less  than  1.5  mm.  in  height,  that  in  which  this  article  is  printed 
(eleven  point),  should  ever  be  used,  the  fatigue  increasing  rap- 
idly even  before  the  size  becomes  as  small  as  this.  The  intensity 
of  illumination  is  apparently  of  little  consequence  within  the  lim- 
its of  daylight  in  well  lighted  rooms.  Very  low  intensities, 
less  than  from  3  to  10  candle-meters,  are  sources  of  even  greater 
fatigue  than  small  type,  and  100  C.  M.  may  be  considered  a  safe 
limit.  Yet  the  illumination  in  German  school  rooms  has  been 
found  to  be  frequently  less  than  2  C.  M.  White  light  rather  than 
yellow  light  should  be  used  for  artificial  illumination.  The  form 
of  the  type  is  of  less  importance  than  the  thickness  of  the  letters. 
White  paper  should  be  used,  though  it  is  possible  that  the 
greater  amount  of  light  reflected  from  pure  white  paper  may 
cause  some  fatigue.  Additional  '  leading  '  or  spacing  between 
the  lines,  is  also  desirable. 


12491 


U^^\^;,'ir-    >n  the  last  date  stamped  o 


ttKTVSESITY  of  CAUFuKi^i^-^ 

AT 

LOS  ANGELSS 

LIBRARY 


^PHIET  BINDER 

^^::^    Syrocuse,  N.    Y. 
Slocklon,  Calif. 


ETduc-Psych. 
Library 

BF 


,,i'/C  SOUTHERN  REGIONAL  LIBRARY 


D     000  432  887 


